Shadow stacks are a control-flow-integrity feature designed to defend against exploits that manipulate a thread's call stack. The kernel first gained support for hardware-implemented shadow stacks, for the x86 architecture, in the 6.6 release; 64-bit Arm support followed in 6.13. This feature does not give user space much control over the allocation of shadow stacks for new threads, though; a patch series from Mark Brown may, after many attempts, finally be about to change that situation.